Avalanche forecast

Bitterroot

West Central Montana Avalanche Center
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Avalanche danger

Fri
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
2 · Moderate
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Out of date — this forecast expired 4 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Stability is poor at mid and upper elevations across the forecast zone. Skiers and riders are likely to trigger 2-3 foot deep avalanches. Remote triggering from above beside and below avalanche slopes is possible.

Pay close attention to signs of instability like collapsing and whumpfing. Stay off and out from under slopes of 30 degrees or more. 

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2.5

    The snowpack has multiple persistent weak layers and all demand respect. Across the Bitterroot and Rattlesnake zones, a layer of buried surface hoar 2 feet down has produced recent avalanches and widespread collapsing. This layer is present on all aspects from mid to upper elevations. 

    Our second persistent weak layer is basal facets and depth hoar. This layer is primarily on northern aspects but also exists in sheltered areas where early season snow from October did not melt. This layer has not been as reactive as the surface hoar. However, snowpit tests show potential for a significant full depth avalanche.

    Stick to low angle terrain below 30 degrees. As the slab above these weak layers becomes more cohesive, the potential for a slab to pull back onto lower angle terrain increases. Be aware of shallow or rocky areas, as these are likely trigger locations for slab avalanches. 

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Winds the past two days have been moderate from the southwest, with strong gusts ideal for transporting new snow. Wind slab exists on north and east aspects with cross loading in gullies and complex terrain. The potential for a larger avalanche exists in areas where the wind slab lies above persistent weak layers. 

    Pay close attention to surface conditions along ridgelines and watch for smooth or rounded features and any hollow-feeling slab. Stay off north and east facing slopes above 30 degrees and give any cornice development a wide berth as cornice often breaks further back than anticipated.

Avalanche Discussion

The northern portion of the forecast zone around Seeley lake was the winner out of the storm on Tuesday, while the southern Bitterroot picked up only a trace of new snow. The North Fork of the Jocko Snotel reported just under an inch of snow water equivalent (SWE), but snow totals range from 3 inches to 8 inches in the town of Seeley Lake. For more information on snow water equivalent, check out this blog post!

In the southern Bitterroot, the cold clear weather has created a new layer of surface hoar on top of the snowpack. We will be watching this as the next storm moves in later in the week. If it becomes buried, we will have another persistent weak layer to track.

Southwest winds and moderate speeds have been ideal for transporting new snow, and yesterday we saw new cornice growth on east aspects around Saddle Mountain. In the Lolo Pass area, observers reported a firm wind slab cracking 6 inches deep under their skis. 

The snowpack across the forecast zone has a very poor structure. The early season snow buried at the bottom of the snowpack has progressed to depth hoar. The further south you go in the Bitterroot, the weaker this depth hoar layer becomes. In the Rattlesnake yesterday, a WCMAC forecaster got an ECTP4 in an extended column test, and a 13/100 end result in a propagation saw test. These results show the potential for a significant slab avalanche out of this layer. Any small avalanche has the potential to step down and trigger a full depth avalanche on deep faceted layers. A deeper snowpack is the only way these faceted layers will round and build strength.

A buried surface hoar layer with a 2-3 foot deep slab above it exists across the Bitterroot and Rattlesnake and has produced recent avalanches. Observers reported widespread collapsing and remotely triggered multiple avalanches in the northern Bitterroot and Rattlesnake. Snowpit tests on the buried surface hoar layer show high propagation potential, with one forecaster in the Snowbowl backcountry reporting an ECTP6.  The same weather that produced surface hoar in Rattlesnake likely produced it in the Seeley Lake area. 

The avalanche hazard at lower elevations between 4000 and 5500 feet is generally low due to minimal snow coverage. The exception to this is the Lolo Pass area, where enough low elevation snow exists to produce an avalanche. The potential exists, however, for an isolated sheltered pocket outside of Lolo Pass to hold enough snow for an avalanche. 

The weak layers in our snowpack are going to be around for a while.  As the slab above the weak layers gains strength, it will become more tempting to move out onto steeper avalanche prone slopes. Remember the big picture. We have a poor snowpack structure with multiple known weak layers and a slab—all the ingredients for a dangerous avalanche. Don't let a lack of results in snowpit tests justify skiing a slope with such poor snowpack structure. Terrain selection is the best way to avoid avalanches today. Stick to low angle terrain under 30 degrees. An inclinometer is an excellent tool for reading slope angles in avalanche terrain. Like any tool, it requires practice to read accurately, but the information it can provide you is life saving. 

Please send us an observation of what you are seeing out there! We will be tracking these weak layers for a while, and all information is useful.

Ski and ride safe!

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